A conventional, electric, submersible pump (ESP) assembly includes an electrical motor and a rotary pump that is used to pump oil or other fluids from a well. The motor has a rotatable rotor that is contained within a stationary stator. The rotors for the submersible pumps are usually disposed in substantially vertical position by virtue of the wells often being vertical. The rotor shafts may be 30 feet or more in length, thus need radial support bearings to stabilize them and reduce vibration.
The bearings that provide radial support for the rotor shaft are often of the fluid film variety. However, fluid film bearings require a side load to provide optimal dynamic stability. Since the rotor shaft is rotating in a vertical position, there is little or no side load being applied to the bearing during operation. This causes instability in the bearings, which can result in excessive motor vibration. Excessive vibration in the bearings can cause the bearing sleeves to break through the lubricant film, resulting in metal-to-metal contact that can lead to premature wear and motor failure.
A typical motor contains an internal lubrication system that circulates lubricant from a reservoir, through a hollow motor shaft, and through passages in the shaft to lubricate the radial support bearings. The circulation occurs by reason of the rotating shaft. Normally, there is no pump, and the pressure due to the rotating shaft is low. The lubricant may also circulate through a heat exchanger and through a particle filter and/or a hygroscopic material to remove heat and contaminants from the lubricant. Although pumps being currently marketed rely on convection circulation of lubricant, prior art patents exist that show internal lubricant pumps for circulating lubricant. The circulation created by the prior art patents would not appear to provide sufficient pressure to create a fluid film between the bearings and the shaft.
Where lateral loading of a component is too low for fluid film stabilization of journal bearings, pressurization of the lubricant may be used. Stabilization occurs when a lubricant is fed into a bearing-component interface at a pressure sufficient to maintain a film between the component and the bearing even when there is minimal loading. While some pressure is developed in an ESP motor designed for lubricant circulation, it is much too low to achieve stabilization of the bearing through fluid film stabilization.